The official, prominent in Her Majesty’s Government, was dashing down Downing Street, heading to an important meeting in early March. Topic: what the hell to do about Vladimir Putin’s Cold Warrior ways and vows of vengeance in Ukraine.

The official was clutching a position paper of talking points, complete with scribbled notes of emphasis. He probably never noticed the photographer he walked past. Never heard the camera click.

And certainly never realized he’d just given Russia’s president a gift of invaluable reassurance. Within hours, the words the camera captured on the official’s paper became big news in Britain and Europe. And far bigger news inside the Kremlin.

Via this unorthodox diplomatic pouch, Putin realized he could launch some sort of face-saving retaliation for the humiliation Russia suffered when Putin’s puppet was suddenly booted out as Ukraine’s president. Probably without disastrous economic consequences.

“U.K. should not support for now trade sanctions or close London’s financial center to Russians,” the official document warned, the BBC reported — a report made authoritative by publishing the photo that showed the document’s words we all could read.

The tip sheet told Putin the European Union countries were considering imposing visa restrictions and travel bans on key Russian figures. (A minuscule price for a face-saving!) But it importantly added that Europe’s leaders should “discourage any discussion of contingency military preparations” and should support “contingency EU work on providing Ukraine with alternative gas if Russia cuts them off.”

That was a key part of how Putin realized he could probably get away with some kind of bold move after the way Ukraine had rejected Russia’s leadership and turned west to woo Europe.

But because Putin also knew it would be disastrous if America and Europe united behind an effort to isolate Russia from the entire global economy, he needed another crucial piece of insider info. He needed to know if Germany — Europe’s mightiest economy and Russia’s top trading partner — was in a mood to clamp on tough sanctions.

Shamefully, but not surprisingly, German Chancellor Angela Merkel wasn’t talking about tough economic sanctions if Russia tried anything. Germany, the grateful consumer at the other end of the Russian natural-gas pipeline that ran conveniently through Ukraine, wanted nothing to upset its economic progress.

Indeed, all the early tough talk was coming from across the pond — warnings of stiff sanctions came from President Barack Obama and Secretary of State John Kerry. But Putin knew Washington tough talk wouldn’t work unless Europe was determined to get tough, too. And so far, London and Berlin were just threatening a few lashes with a wet noodle.

The Kremlin was drawing strength from Europe’s milquetoast mouthings and photogenic papers. They realized London and Berlin didn’t want to undo their cozy economic ties with Russia’s oligarchy.

By then, Putin had settled on what could be Russia’s perfect way to gain a virtually pain-free victory. Crimea! It was always Russian in its heart, even after Nikita Khrushchev gave it to Ukraine. The plot was hatched: insert a handful of military troops — wink-wink, with no identifying insignias on their shirts — to signal Crimeans they can finally come back. Then hold a quickie referendum; Crimea will joyfully vote itself back to the bad old days.

That’s how Putin figured he could override the memory chip of the shame of being rejected by Ukraine in favor of Europe. The former KGB colonel would defeat the world’s greatest democracies with their own super-weapon — a democratic election, of all things!

For weeks, U.S. and European diplomats had told each other what they needed to do was build a diplomatic and financial off-ramp Putin could use to get away from his humiliation of the crisis in Kiev.

But to the surprise of the pinstriped set, Putin was already building his own off-ramp — the Crimea cutoff. And in the end, the democracies of the West couldn’t convincingly argue against the will of the people. Especially when it already was, as the Russians love to say, a fait accompli.